Drilling oil wells is an expensive operation both in the equipment used and in the risks run. One of these risks is the risk of the drilling tool becoming jammed downhole, thereby jamming an entire drilling string which may be very expensive.
When such jamming occurs, the first operation normally performed consists in applying a large traction force to the drilling string from the surface in an attempt to unjam the tool. However the traction force which can be applied is limited by the risk of breaking the string or some of its components. If such traction is ineffective, the next procedure is to back off by attempting to disconnect two drillstems at as low a point as possible. (This conventional operation consists initially in applying a screwing torque in order to equalize the extent to which successive connections between stems are tightened, then in applying an unscrewing torque which is slightly less than the torque needed to disconnect the stems, and then in exploding a charge level with the connection to be unscrewed while the unscrewing torque is maintained. In this way, the portion of the drilling string above the unscrewed connection can be raised to the surface). Thereafter a string fitted with a hammer slide is lowered to apply axial traction shocks in an attempt to release the tool. If this operation is unsuccessful, a further backing off operation is performed and the unrecovered equipment is abandoned downhole.
The next operation consists in casting a cement plug. These operations end up with the loss of 200 to 300 meters of drilling string (heavy stems, MWD measurement equipment, motors, turbine, tools, . . . ), and 350 to 400 meters of drilling, not counting various side-tracking operations that will then be required to get out from the drilled hole.
This risk is a major risk when passing through unstable, cracked or heterogeneous ground.
The present invention seeks to avoid, or at least to minimize, the above-mentioned risks of loss.